I can't find the Master Jaw vertex group using the Rigify addon
Rigify is a quite complicated rig, in which a lot of bones are "control" or "machinary" bones, so they don't deform the mesh (they don't have the "deform" option activated, so no associated vertex group is created when parenting). 

All and the only bones who deform the mesh are stored into a generally hidden bone layer, and have the "DEF" suffix on their names.

So, probably the vertex group you're looking for is called "DEF-jaw".

Of course, after generating the rig, if you know what you are doing you can modify all of theese properties.